1. Field of the Invention
The present invention relates to an on-vehicle radar or, in particular, to a method of determining the axial deviation of the on-vehicle radar using the track of a stationary object.
2. Description of the Related Art
The use of a scanning-type radar on a vehicle requires that the axis of the direction in which an automotive vehicle is running straight coincides exactly with the center line of beam scanning. A deviation, if any, of the axis due to the vibration or the positional variation of the radar while the vehicle is running would result in an erroneous detection of the position of an object such as another vehicle running ahead. While the vehicle is running, therefore, the axial deviation is detected constantly and corrected (Japanese Unexamined Patent Publication No. 2001-166051) (Patent Document 1).
FIGS. 1 to 2B show an example of the conventional method of axial deviation.
FIG. 1 shows an example of a straight road having lanes 12, 14 on the left and right sides of a white center line 13 with guard rails 11 and 15 providing stationary objects on the sides of the road on which a vehicle 10 is running along the left lane 12.
FIG. 1 also shows both a radar detection area 18a without any axial deviation of the radar mounted on the vehicle 10 and a radar detection area 18b having a rightward axial deviation (at axial deviation angle θ). Reference numeral 16 designates the axis of the direction in which the vehicle 10 is running straight, and numerals 17a, 17b the scanning center axes of the beams of the radar detection areas 18a, 18b, respectively.
FIG. 2A shows an example of detection of a stationary object without any axial deviation.
In this case, the beam is scanned in the radar detection area 18a, and the distance to the guard rails 11, 15 providing stationary objects is detected a plurality of times for each beam angle (Φi) within a predetermined time length. The result of detection is plotted on the x-y coordinate with the current position of the vehicle 10 as the origin, the ordinate representing the straight distance covered and the abscissa the left and right lateral positions. FIG. 2B is the same as FIG. 2A except that in FIG. 2B, the beam is scanned in the radar detection area 18b which has developed an axial deviation (axial deviation angle θ).
In FIGS. 2A, 2B, each boundary between the road and the guard rails 11, 15 providing the stationary objects (hereinafter referred to as the calculated stationary object line), calculated using a predetermined algorithm based on the detection data, is illustrated by a thick solid line. In the state free of axial deviation shown in FIG. 2A, the calculated stationary object line is parallel to the axis 16 of the direction in which the vehicle is running straight ahead. In the state which an axial deviation has developed, as shown in FIG. 2B, on the other hand, the calculated stationary object line is tilted by the axial deviation angle θ from the axis 16 of the direction in which the vehicle 10 is running straight ahead. As a result, the axial deviation of the radar mounted on the vehicle 10 can be detected and, if necessary, the axial deviation angle θ is corrected by the axial deviation adjusting mechanism in the vehicle 10.
An axial deviation may be detected by the method described above, however, it may also be detected in accordance with the environment or mode in which the vehicle is running as well as due to the actual axial deviation of the radar. If, for example, a guard rail is also formed in the side strip constituting a part of the road, or a tunnel entrance narrower than the width between the guard rails exists ahead, or the vehicle changes the lane on a two-lane road, the data on the stationary objects detected a plurality of times within a predetermined time length and accumulatively plotted and displayed are not as simple as the linear form shown in FIGS. 2A, 2B, but have a complicated shape with some redundancy containing a plurality of rises therein.
In the case where the calculated stationary object line is determined from the detection data having such a complicated shape, there is a problem that the calculation accuracy is reduced and the radar, even if correctly installed (with the radar detection area 18a), may detect an erroneous axial deviation (with the radar detection area 18b). Another problem is that automatic adjustment of the radar angle to compensate for the erroneously detected axial deviation would adversely affect the control of the running vehicle.